Iterative method of decoding a received signal

ABSTRACT

An iterative method and a device ( 20 ) for decoding received signals ( 21 ) transmitted in data frames via various channels (A, B). In order to be able to utilize the computing capacity of digital signal processors (DSPs) as efficiently as possible, it is proposed that, starting at a first channel (A; B), the quality of the decoded signal ( 33 ) transmitted via the first channel (A; B) is checked following every iteration (A 1 , . . . , A 5;  B 1 , . . . , B 3 ) and switchover takes place to at least one further channel (B; A) if a specifiable switchover condition exists. The switchover condition may, for example, be a specifiable quality of the decoded signal ( 33 ) transmitted via the channel under consideration. To determine the quality of the decoded signal ( 33 ) a cyclic redundancy check (CRC), in particular, is proposed following every iteration (A 1 , . . . , A 5;  B 1 , . . . , B 3 ).

The present application is a divisional application of application Ser.No. 10/114,281, filed Apr. 3, 2002, and which is hereby incorporatedinto the present application by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an iterative method of decoding areceived signal transmitted in data frames.

The invention relates, in addition, to an iterative method of decodingreceived signals transmitted in data frames via various channels.

Furthermore, the present invention relates to a device for decodingreceived signals by means of an iterative method.

The invention also relates to a device for receiving received signalstransmitted in data frames with at least one device for decoding thereceived signals by means of an iterative method.

The invention furthermore relates to a base station of a radiotelecommunications system having a multiplicity of mobile-radio units,having at least one further base station, the base stations being inradio contact with the mobile-radio units, and at least one controldevice for controlling the radio telecommunications system that is incontact with the base stations.

Finally, the present invention relates to a radio telecommunicationssystem having a multiplicity of mobile-radio units, having a pluralityof base stations that are in radio contact with the mobile-radio units,and having at least one control device for controlling the radiotelecommunications system that is in contact with the base stations. Theinvention is based on a priority application EP 01 440 100.4 which ishereby incorporated by reference.

Decoding methods of the type mentioned at the outset are used in theprior art, for example in decoding devices of base stations of radiotelecommunications systems. FIG. 2 shows a block diagram of a knownso-called turbo decoder, such as is used, for example, for decoding achannel in a node B of UMTS (Universal Mobile Telecommunications System)radio telecommunications systems. More detailed items of information onthe known turbo decoder can be found in 3-GPP (3^(rd) GenerationPartnership Project) specification 25.212 V.3.4.0, to which reference isexpressly made. A received signal applied to the turbo decoder isdenoted by the reference symbol 1. The input signal is transmitted indata frames according to a CDMA (code division multiple access) method.It is first fed to a so-called unpuncturing unit 2, which matches thereceived signal 1 transmitted in the data frames to specified dataframes of the decoder. This is done, for example, by adding bits omittedin the frame of the data transmission. The added bits have the amplitudezero. They are treated in the decoding frame as bits having a lowreliability and weighted low according to the amplitude. Theunpuncturing is part of a so-called rate matching. Applied to theunpuncturing unit 2 are an output signal e and two redundancy signalsy₁, y₂ for the purpose of error correction.

The output signal e and the first redundancy signal y₁ are fed to afirst so-called constituent decoder 3 having a so-called soft output atthe output 4. A soft output means that the entire bandwidth of realnumbers can be applied to the output 4 instead of, as in the case of theso-called hard output, where only +1, −1 (physical bits) or 0, 1 (logicbits) may be applied at the output. As a result, errors in the decodingof a transmitted received signal subject to interference are reduced. Inthe case of the soft output, the bits have various amplitudescorresponding to their reliability and are weighted according to theiramplitude. Used as constituent decoders are, typically, decoders of theMAP (maximum a posteriori) type or, for example of the LogMAP,MaxLogMAP, SOVA or Viterbi types.

The output 4 of the decoder 3 is fed to an interleave unit 5, whichrearranges the bits within the data frames so that originally adjacentbits are as far away from one another as possible. In this connection,it is assumed that errors in the transmission of the received signalaffect not only individual bits but, as a rule, a multiplicity ofconsecutive bits. Whereas the decoding of a data frame containingoccasionally occurring defective bits is possible in a relativelyproblem-free manner, decoding of a data frame having a multiplicity ofconsecutive defective bits is very expensive or even impossible. Forthis reason, the bits are rearranged by the interleave unit 5 in orderto generate a data frame containing defective bits that occur onlyoccasionally and that can readily be decoded despite a transmissionerror that occurs in an accumulated manner and that affects amultiplicity of consecutive bits.

The output of the interleave unit 5 is fed, together with the secondredundancy signal y₂, to a second constituent decoder 6. From the pointof view of structure, the latter corresponds to the first constituentdecoder 3. According to the prior art, the output of the secondconstituent decoder 6 is regularly fed back eight times via a feedback 7and a de-interleave unit 8 disposed therein to the input of the firstconstituent decoder 3. After eight iterations have taken place, theoutput of the second constituent decoder 6 is emitted via a thresholddetector 9 and a further de-interleave unit 10. The output 11 of thefurther de-interleave unit 10 is the decoded signal (the so-calledestimated hard bits).

Turbo decoding involving eight iterations of a received signal that istransmitted via a 384 kbit/s data channel requires a computing capacityof 78×10⁶ instructions/s (78 Mips). At a clock rate of 180 Hz and 60%capacity utilization, a conventional digital signal processor (DSP) ofthe TigerSHARC type can make available about 108×10⁶ instructions/s (108Mips). Consequently, according to the prior art, a maximum of 384 kbit/sdata channel can be decoded by a DSP.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to reduce the loadingof a digital signal processor for decoding received signals transmittedin data frames.

Proceeding from the iterative method of decoding a received signaltransmitted in data frames, the invention proposes, to achieve thisobject, that the quality of the decoded signal is checked followingevery iteration and the method is terminated if the decoded signal has aspecifiable quality.

According to the invention, therefore, it is proposed not always toperform a specified number of iterations in an iterative decodingmethod. If the decoded signal has a specifiable quality even after alesser number of iterations than the specified number, the method ofdecoding the part of the received signal contained in a data frame isalready terminated, according to the invention, before all the specifiediterations have been run.

The method according to the invention is able to reduce considerably theloading of digital signal processors (DSPs) for the decoding of receivedsignals transmitted in the data frame. Experiments in a UMTS radiotelecommunications system have shown that, with an assumed meaninterference of the received signal of about 1 dB, the plurality of dataframes is already error-free even after two iterations or less, whereas,with a data frame error rate (block error rate, BLER) of about 1% orless, there is talk of freedom from error after eight iterations. In thecase of turbo decoders that are used in a node B of a UMTS radiotelecommunications system, the method according to the invention canreduce the capacity utilization of the DSPs by about 70%, given anassumed signal-to-interference ratio (SIR) of the received signal ofabout 1 dB.

The marked reduction of the capacity utilization of the DSPs as a resultof the method according to the invention has various advantages. On theone hand, lower performance and, consequently, markedly less expensiveDSPs can be used in a decoding device because of the reduced capacityutilization. In accordance with another approach, the samehigh-performance DSPs can also be used as before, but the DSPs are givenother tasks at the times at which they are not decoding the receivedsignal, that is to say are in a so-called idle state, and canconsequently reduce the loading of other processors in the decodingdevice.

In this connection, for example, the design of the decoding device as aso-called packet machine having its own control unit, a so-calledscheduler, would be conceivable. The scheduler allocates the decoding ofa certain data frame to a DSP in the decoding device that is currentlyin an idle state. In the case of a decoding device constructed as apacket machine, the number of DSPs provided in the decoding device fordecoding received signals can be reduced, but, at the same time, thefull functionality of the decoding device is retained.

In accordance with an advantageous development of the present invention,it is proposed that a cyclic redundancy check (CRC) is performed forevery decoded data frame following every iteration and the method isterminated if the decoded data frame is error-free. CRC is a method ofchecking the freedom from error of a signal transmitted digitally via acommunications link. In a transmitting unit, CRC bits are generated byprocessing the data to be transmitted in accordance with a CRCalgorithm. The result of the CRC algorithm is stored in so-called CRCbits that are transmitted together with the data to be transmitted. Thereceiving unit performs the same CRC algorithm on the received data andcompares the result of the CRC algorithm with the received CRC bits. Ifthey do not match, a transmission error has occurred and the receivingunit can initiate, for example, a repeat transmission of the data. Asused in decoding devices of a node B of a UMTS radio telecommunicationssystem, turbo decoders have, as standard, a facility for performing aCRC on the received signal. In accordance with the present development,use is made of this facility for every decoded data frame followingevery iteration that is run in the framework of the decoding method. Ifthe result of the CRC algorithm matches the CRC bits, the method isterminated.

Proceeding from an iterative method of decoding received signalstransmitted in data frames via various channels, it is proposed, as afurther achievement of the object, that, starting with a first channel,the quality of the decoded signals transmitted via the first channel arechecked following every iteration and a switchover takes place to atleast a further channel if a specifiable switchover condition exists.

The specifiable switchover condition may, for example, exist if thedecoded signal transmitted via the channel under consideration has aspecifiable quality. In this case, it is proposed, in accordance with adevelopment of the present invention, that a switchover takes place toat least one further channel. As a result, the same DSP can be used todecode a first data frame that is transmitted via the first channel andat least one further data frame that is transmitted via the at least onefurther channel. How many different data frames can be decoded within aninterval of time under consideration (so-called transmission timinginterval frame, TTI frame) by a DSP depends, on the one hand, on thecomputing capacity of the DSP and, on the other hand, on the number ofiterations that have to be performed to decode preceding data frameswithin the same TTI frame.

In accordance with a preferred embodiment of the present invention, itis proposed that a cyclic redundancy check (CRC) is performed for everydecoded data frame of the channel under consideration following everyiteration and a switchover takes place to the at least one furtherchannel if the decoded data frame transmitted via the channel underconsideration is error-free.

Advantageously, a switchover takes place to a further channel althoughthe decoded signal transmitted via the channel under consideration doesnot have the specifiable quality if a specifiable first number ofiterations has been performed for a data frame of the channel underconsideration. If, for example, a maximum of eight iterations isprovided for decoding a data frame transmitted via a certain channel,the method is terminated after eight unsuccessful iterations. Thistermination of the decoding of the data frame transmitted via thechannel under consideration prevents an excessive computational loadingof the DSP, for example in the event of particularly severe interferencein the input signal, and makes it possible to use computing capacitiesstill remaining on the DSP within the TTI frame for decoding data framestransmitted via further channels. The termination of the decoding forthe data frame transmitted via the channel under consideration meansthat said data frame can either not be decoded at all because, forexample, it has interference within the framework of the radiotransmission exceeding the so-called Shannon theorem, or cannot bedecoded with acceptable effort. The Shannon theorem is an upper limit ofthe degree of interference in a signal at which interference suppressionin the signal is still just theoretically possible.

Instead of decoding first one channel with a plurality of consecutiveiterations and only thereafter switching over to further channels inorder to decode them in turn in each case individually in consecutiveiterations, it is also conceivable to switch over after every iterationto a further channel of a group of channels and to terminate thedecoding of a channel if the decoded signal transmitted via the channelunder consideration has a specifiable quality. In that case, aswitchover takes place only between the remaining channels of the group.Accordingly, it is proposed, as a further advantageous development ofthe present invention that, starting with a first channel, switchoverbetween the first channel and at least one further channel takes placefollowing an iteration cyclically in each case for a certain number ofiterations until the decoded signal transmitted via the channel underconsideration has a specifiable quality.

In accordance with a preferred embodiment of the invention, it isproposed that for every decoded data frame of the channel underconsideration, a cyclic redundancy check (CRC) is performed followingevery iteration and switchover takes place between the first channel andthe at least one further channel until the decoded data frametransmitted via the channel under consideration is error-free.

Preferably, following every iteration, a switchover takes placecyclically in each case for an iteration between the first channel andthe at least one further channel.

In accordance with a further preferred embodiment of the presentinvention, it is proposed that the method is terminated if the decodedsignal transmitted via a specifiable further channel has a specifiablequality. If a DSP, for example, is used to decode a maximum of fivechannels, the decoding method is terminated if the data frametransmitted via the fifth channel is also error-free. The DSP is then inthe idle state and is available for other calculations.

Preferably, the method is terminated although the decoded signaltransmitted via the channel under consideration does not have thespecifiable quality if a specifiable second number of iterations hasbeen performed for the data frames of the first and the furtherchannels. If in total a maximum of ten iterations are available fordecoding data frames transmitted via various channels within a TTIframe, the method is terminated after ten iterations, the data frame ofa first channel having been successfully decoded after six iterations,but the four iterations remaining within the TTI frame have not beensufficient for decoding a further data frame.

The method according to the invention can start decoding the data frametransmitted via a channel at any desired channel. It has been found,however, that the efficiency of decoding can be increased markedly yetagain if certain criteria are taken into account in selecting the firstchannel. Therefore, in accordance with another advantageous developmentof the present invention, it is proposed that of a specifiable number ofchannels, that channel is selected as first channel via which the signalis transmitted with the best quality factor. In this connection, it isassumed that the decoding of such a signal having a high quality factorrequires only very few iterations until the decoded signal has thespecifiable quality. The relatively long computation time of the DSPremaining within the TTI frame can then be fully used to decode a dataframe transmitted via a further channel. The probability of a successfuldecoding of the further data frame is relatively high, even if itcontains strong interference and requires a relatively large number ofiterations for decoding since, because of the rapid decoding of thefirst data frame within the TTI frame, a large computing capacity isstill available in the DSP.

It is conceivable to determine the quality factor of the signals of thespecifiable number of channels on the basis of an analysis of theamplitude variations at the input of the decoding device. In accordancewith a preferred embodiment of the invention, it is proposed that thequality factor of the signals of the specifiable number of channels isdetermined on the basis of a signal-to-interference ratio (SIR) of thesignals.

In accordance with a further preferred embodiment of the invention, itis proposed that the quality factor of the signals of the specifiablenumber of channels is determined on the basis of a bit error rate ofbits in a control channel of the signals. Preferably, the quality factorof the signals of the specifiable number of channels is determined onthe basis of a bit error rate of pilot bits in a dedicated physicalcontrol channel (DPCCH) of the signals. A data channel of a UMTS radiotelecommunications system is described as a data channel (DCH). The datachannel comprises a dedicated physical data channel (DPDCH) and thededicated physical control channel (DPCCH). This involves the individualchannels assigned to the terminals. The DPDCH is subdivided yet againinto a dedicated traffic channel (DTCH) and a designated control channel(DCCH). The DPDCH is transmitted about once every 20 to 40 msec. TheDPCCH contains the pilot bits for synchronization and further bits. TheDPCCH is transmitted approximately 1,500 times per second. The pilotbits are an important index of the quality factor of the transmissionsignals.

Proceeding from the device for decoding input signals by means of aniterative method, it is proposed, as yet a further achievement of theobject of the invention, that the decoding device has means forperforming the method according to the invention.

In accordance with an advantageous development of the present invention,it is proposed that the decoding device has at least two constituentdecoders connected in series, an interleave unit disposed in each casebetween two constituent decoders, a feedback of an output of the lastconstituent decoder at an input of the first constituent decoder and ade-interleave unit disposed in the feedback. Such a decoding device isdescribed as a turbo decoder and is used, in particular, in a receivingdevice in a node B of a UMTS radio communications system.

Proceeding from the device for receiving received signals transmitted indata frames of the type mentioned at the outset, it is proposed, as afurther achievement of the object of the invention, that the decodingdevice has means for performing the method according to the invention.Such a receiving device is present in the case of a UMTS radiotelecommunications system in a node B for receiving signals of themobile-radio units (so-called uplink).

Proceeding from the base station of a radio telecommunications system ofthe type mentioned at the outset, it is proposed, as yet a furtherachievement of the present invention that the base station has at leastone device according to claim 16 for receiving a received signaltransmitted in data frames with a decoding device for decoding thesignal by means of an iterative method. Such a base station is describedin a UMTS radio telecommunications system as node B.

Finally, proceeding from the radio telecommunications system of the typementioned at the outset, it is proposed as an achievement of the objectof the present invention that at least one of the base stations of theradio communications system is constructed as a base station accordingto claim 17. The mobile-radio units of the radio telecommunicationssystem are, for example, constructed as mobile telephones. In a UMTSradio telecommunications system, the base stations are described as nodeB and the control device as radio network controller (RNC).

In accordance with an advantageous development of the present invention,it is proposed that a mobile-radio unit is in radio contact with aplurality of base stations, the base stations that are in radio contactwith the mobile-radio unit transmit the quality factor of the radio linkto the control device, the control device communicates items ofinformation about the quality factor of the radio links to the basestations and the base stations use said items of information inperforming the method according to the invention. The state in which amobile-radio unit is in radio contact with a plurality of base stationsis also described as soft handover. As a result of the fact that theitems of information about the quality factor of the radio links to allthe base stations with which the mobile-radio unit is in contact isavailable in the individual base stations, every individual base stationknows whether the radio link with which it is in radio contact with themobile-radio unit tends to be good or tends to be bad. Said items ofinformation can then be used in the individual base stations within theframework of performing the method according to the invention at variouspoints. Thus, a base station may, for example, decode the signalreceived via the radio link from the mobile-radio unit with higherpriority if it knows that it is maintaining the radio link having thebest quality factor to the mobile-radio unit. Equally, a base stationthat is maintaining a radio link having a quality factor that tends tobe poorer to the mobile-radio unit may treat the decoding of the radiosignal with a priority that tends to be lower.

In accordance with an advantageous development of the present invention,it is proposed that the base stations use the items of information aboutthe quality factor of the radio links to select the first channel from aspecifiable number of channels. The radio link via which the receivedsignal is transmitted with a particularly high quality factor istherefore decoded with the highest priority. If the DSP then still hascomputing capacity free, lower-priority received signals can be decoded.

According to a preferred embodiment of the present invention, it isproposed that the base stations use the items of information about thequality factor of the radio links to select the sequence of furtherchannels from a specifiable number of channels.

In accordance with a further preferred embodiment of the presentinvention, it is proposed that the base stations use the items ofinformation about the quality factor of the radio links to select theinstant in time of a change to a further channel.

Finally, in accordance with yet a further preferred embodiment of theinvention, it is proposed that the base stations use the items ofinformation about the quality factor of the radio links to select theinstant in time of a termination of the method.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, possible applications and advantages of the inventionemerge from the description below of the exemplary embodiments of theinvention that are shown in the drawing. In this connection, all thefeatures described or presented separately or in any combination formthe subject of the invention, regardless of their inclusion in thepatent claims or of their back-reference and also regardless of theirformulation or presentation in the description or in the drawing.

In the drawing:

FIG. 1 shows a block diagram of a decoding device according to theinvention;

FIG. 2 shows a block diagram of a decoding device known from the priorart;

FIG. 3 shows a block diagram of a radio telecommunications systemaccording to the invention;

FIG. 4 shows a flowchart of a decoding method according to theinvention;

FIG. 5 shows a method according to the invention in accordance with afirst preferred embodiment;

FIG. 6 shows a method according to the invention in accordance with asecond preferred embodiment; and

FIG. 7 shows a method according to the invention in accordance with athird preferred embodiment.

FIG. 1 shows a block diagram of a decoding device according to theinvention in accordance with a preferred embodiment. The decoding deviceis denoted in its entirety by the reference symbol 20. An input signaltransmitted in data frames and applied to the decoding device 20 isdenoted by the reference symbol 21. The input signal 21 is, for example,an uplink signal of a mobile radio unit of a radio telecommunicationssystem that is received in a base station of the radiotelecommunications system. The input signal 21 is applied to anunpuncturing and demultiplexing unit 22, which generates an outputsignal e and two redundancy signals y₁, y₂ for the purpose of errorcorrection. The output signal e and the first redundancy signal y₁ areapplied to a first constituent decoder 23, which is constructed, forexample, as a Viterbi decoder. It may, however, also be constructed as aso-called MAP, Log-MAP or MaxLogMAP decoder. An output 24 of the firstdecoder 23 is fed to an interleave unit 25. An output of the interleaveunit 25 and the second redundancy signal y₂ are fed to the input of asecond constituent decoder 26, which may be constructed, for example, asa Viterbi decoder. A decoded signal 33 is fed back again via a feedback27 and a de-interleave unit 28 disposed therein to the input of thefirst decoder 23. To this extent, the decoding device 20 according tothe invention is similar to a decoding device known from the prior artshown in FIG. 2.

In contrast to the known decoding device, the decoding device 20according to the invention has, however, a device 32 for performing acyclic redundancy check (CRC) following every iteration run. The qualityof the decoded signal 33 can be checked with the aid of the CRC. If theCRC reveals that the data frame under consideration of the decodedsignal 33 is not error-free and that the decoded signal 33 does nottherefore have the specified quality (as yet), the decoded signal 33 isfed back via the feedback 27 to the input of the first decoder 23 and isdecoded once again within the framework of a further iteration.

If the data frame of the decoded signal 33 is, however, error-free, thedecoded signal 33 is emitted as estimated hard bits 31 via a thresholddetector 29 for converting the soft output of the decoder 26 into a hardoutput and a further de-interleave unit 30.

The decoding device 20 according to the invention is also described as aturbo decoder. Such a turbo decoder can be used in a base station node Bof a radio telecommunications system 40 (cf. FIG. 3). The radiotelecommunications system shown in FIG. 3 complies with the UMTSstandard and comprises a multiplicity of mobile-radio units MF₁ toMF_(m) and MF_(m+1) to MF_(m+n), a multiplicity of base stations nodeB₁, node B₂ that are in radio contact with the mobile-radio units MF. Inaddition, the UMTS radio telecommunications system 40 comprises acontrol device constructed as radio network controller (RNC) forcontrolling the radio telecommunications system 40. The control deviceRNC is connected to the base stations node B1, node B2. The radiotelecommunications system 40 can be expanded almost as desired (cf.broken lines in FIG. 3; node B_(i), RNC_(j)).

The decoding device 20 according to the invention shown in FIG. 1 isimplemented on a digital signal processor (DSP). Depending on the DSPused, only a certain number of iterations can be performed within a timeperiod under consideration (so-called transmission timing intervalframe, TTI frame). In the present exemplary embodiment, a DSP of theTigerSHARC type having a clock rate of 180 MHz is used that can performten iterations within a TTI frame. Following the TTI frame, the nextdata frame of the subsequent TTI frame of the received signal 21 isalready present for decoding at the input of the decoding device 20.

With a signal-to-interference ratio of the received signal 21 of about 1dB, the device according to the invention can achieve on average about a70% saving in computing effort for the individual DSPs. This saving incomputing effort is achieved in that a full eight iterations are notperformed, as is standard, in the case of the decoding device 20, butthe number of iterations is determined as a function of need. If, forexample, a received signal 21 is already error-free after twoiterations, the decoding can be terminated without quality loss eventhen, that is to say before running all eight iterations. Afterterminating the decoding, the DSP is in a so-called idle state and isavailable for other tasks, for example for reducing the loading on otherDSPs. According to the invention, three strategies are proposed for abetter capacity utilization of the DSPs of a decoding device. Theproposed strategies are denoted as Z, W and Z⁺ strategies. They areexplained below in detail.

FIG. 4 describes a flowchart of a decoding method according to theinvention for the so-called Z-strategy (cf. FIG. 5). The methodaccording to the invention starts in a function block 50. The so-calledunpuncturing is then performed in a function block 51. This function isundertaken by the unpuncturing and demultiplexing unit 22. Theunpuncturing is part of a rate matching and essentially comprises(expressed in simplified form) the insertion of bits omitted in theframe of the radio transmission into the data frame with an amplitudezero. In a function block 52, a first decoding of the input signal 21performed by the first decoder 23 is then executed.

In a function block 53, the output signal 24 of the first decoder 23 isrearranged so that bits originally adjacent in the data frame aredisposed as far away from one another as possible. This function isexecuted by the interleave unit 25. Although the error rate of thereceived signal 21 transmitted in the data frame under considerationremains the same as a result of rearranging the bits in the functionblock 53, defective bits are disposed so as to be distributed over theentire data frame. As a result, a possible error correction can bedecisively improved in the subsequent second decoder 26. In a downstreamfunction block 54, the further decoding executed in the second decoder26 then takes place.

In a scanning block 55, a cyclic redundancy check (CRC) is executed overa decoded signal 33 contained in the data frame under consideration andwhether the decoded signal 33 is error-free or not is checked. If thedecoded signal 33 is error-free (CRC OK), the decoding method of thedata frame transmitted via the channel under consideration can alreadybe terminated after this first iteration. If the decoded signal 33 isnot, however, error-free (CRC NOK), branching takes place to a scanningblock 56 in which a check is made as to whether a specifiable firstnumber of iterations, in the present exemplary embodiment eightiterations, has already been executed for the data frame underconsideration of the channel under consideration. If that is so, thedecoding of the data frame under consideration is terminated for thechannel under consideration after the eight iterations already executedand a branching is carried out to scanning block 59, although thedecoded signal 33 transmitted via the channel under consideration is noterror free. Otherwise, branching is made to a further scanning block 57in which whether, within a time period under consideration, aspecifiable second number of iterations, in the present exemplaryembodiment ten iterations, has been executed in total for the dataframes of all the channels under consideration. If that is so, thedecoding is likewise terminated for the data frame under considerationof the channel under consideration and branching takes place to thescanning block 59.

Otherwise, branching takes place to a function block 58 in which therearrangement, executed in the function block 53, of the bits of thedata frame under consideration is nullified. This function is undertakenby the de-interleave unit 28 in the feedback 27.

Branching then again takes place to the function block 52, in which afurther decoding of the signal 33 already decoded by means of aniteration takes place in the first decoder 23. Iterations for the dataframe under consideration of the channel under consideration areexecuted until the decoded signal 33 is error-free (scanning block 55),until eight iterations have been carried out for the data frame underconsideration (scanning block 56) or until ten iterations have beenexecuted in total (scanning block 57), within the TTI frames underconsideration, for all the data frames of all the channels underconsideration.

Whether ten iterations have already been executed in total for all thedata frames of all the channels under consideration is checked inscanning block 59. If so, the method has to be terminated in a functionblock 60 since data frames of the next TrI frame are already present fordecoding at the input of the decoding device 20. Otherwise, branchingtakes place to a scanning block 61 in which whether data frames ofchannels allocated to this DSP have been decoded is checked. If the dataframes of all the channels have been decoded, the method is likewiseterminated. Otherwise, branching takes place to a function block 62 withwhich the soft output of the second decoder 26 from the function block54 is converted into a hard output. This is either the physical bits 1,−1 or the logic bits 0, 1. This function is implemented by thethreshold-value generator 29. Then, the rearrangement of the bitscontained in the data frame under consideration from the function block53 is nullified in a function block 63. This function is implemented inthe de-interleave unit 30 of the decoding device 20. The estimated hardbits 31 are then emitted in a function block 64. Finally, a switchovertakes place to a further channel in a function block 65. Branching thentakes place again to the function block 51 and the method according tothe invention is run again for the further channel.

FIG. 5 shows the so-called Z-strategy. The time interval underconsideration that is available to the DSP for decoding the inputsignals transmitted in data frames via the channels (TTI frame) isdenoted by the reference symbol T. The channels A and B are assigned tothe DSP. At the start of the time interval T under consideration, thedecoding device 20 is initialized, which initialization comprises, forexample, the initialization of a so-called Rake receiver, such as isused to receive code division multiple access (CDMA) radio signals, andthe initialization of the interleave units 28, 30. The time needed forinitialization is denoted by t_(init). The Z-strategy shown in FIG. 5starts at channel A with the decoding of the data frame underconsideration. A1 to A5 denote the consecutively executed iterations fordecoding the received signal 21 transmitted in the data frame underconsideration of the channel A. After five iterations, the data frameunder consideration is error free at the instant in time t_(A). Thecomputing capacity still available on the DSP within the TTI frame T issufficient for five further iterations of a decoding. The computingcapacity is used to decode a data frame of the channel B. Switchovertakes place to the channel B and the data frame under consideration isdecoded in a plurality of iterations B1 to B3. Following threeiterations, at the instant in time t_(B1), the decoded signal 33transmitted in the data frame under consideration of channel B iserror-free. For the remaining two iterations, the DSP is free, i.e. inan idle state.

The DSP is then available again for the subsequent TTI frame T fordecoding further data frames of the channels A, B. At the start of thetime interval T under consideration, the decoding device 20 is againinitialized for the time period t_(init). This time, the decoding methodstarts at a data frame transmitted via the channel B. The receivedsignal 21 transmitted via the data frame under consideration iserror-free after four iterations at the instant in time t_(B2).Switchover then takes place to the channel A and the decoding of thedata frame A starts with the first iteration A1.

FIG. 6 shows a second preferred embodiment of the method according tothe invention. The method shown in FIG. 6 is described as W-strategy.The W-strategy can prevent a disadvantage of the Z-strategy, namelythat, if the decoding is started of a received signal having relativelysevere interference, the decoding of the relevant data frame requiresalmost all the iterations available within the TTI frame T and only afew iterations are still available for data frames of the furtherchannel. If the data frame of the further channel contains interferencebeyond a very small extent, the remaining iterations are with highprobability not sufficient for decoding the data frame of the furtherchannel until error-free. If, for example, in the Z-strategy, thedecoding of the data frame of the channel A requires a full eightiterations (the worst case being assumed), only two iterations areavailable for decoding the further data frame of the channel B. Thisdisadvantage is eliminated by the strategy described.

In the W-strategy, switchover takes place (as is evident from FIG. 6) tothe other channel after every iteration. This has the result that (toreturn to the worst case described above) at least five iterations areexecuted for the data frame of the further channel even if the dataframe of the first channel is subject to severe interference. Theprobability that a data frame is decoded after five iterations so thatit is error-free is substantially higher in the case of the W-strategythan after only two iterations in the case of the Z-strategy. It is,however, to be noted that the Z-strategy is superior to the W-strategyin certain cases. If, for example, two channels are assigned to a DSPfor decoding and six iterations are needed in each case for decoding thedata frames of the two channels with a maximum of ten iterationsavailable, this has the result in the case of the W-strategy that bothdata frames cannot be decoded until error-free and have consequently tobe discarded. On the other hand, with the Z-strategy, at least one ofthe two data frames is decoded until error-free.

In the exemplary embodiment from FIG. 6, an initialization of thedecoding device 20 is executed again at the start of the TTI frame Tunder consideration for the time period t_(init). The decoding of thedata frame of the channel A is then started. After the first iterationA1, switchover takes place to the channel B and the decoding of the dataframe of the channel B is started. Switchover again takes place after aniteration B1 to the channel A, where the decoding of the data frame A isinterrupted with the second iteration A2. In this way, switchover takesplace cyclically between the channel A and the channel B for everyiteration following every iteration until a decoded signal 33transmitted via the channel A or B under consideration is error free. Inthe present exemplary embodiment, the decoded signal transmitted via thechannel B is error-free after three iterations at the time instantt_(B). Switchover then again takes place to the channel A and theremaining computing capacity of the DSP is fully utilized for thedecoding of the data frame transmitted via the channel A. The data frameof the channel A is error-free after two further iterations, that is tosay after five iterations in total, at the time instant t_(A1). The DSPis then in the idle state for a further two cycles.

To decode the subsequent data frames of the channels A, B, the decodingof the data frame transmitted via the channel B is started andswitchover again takes place for every iteration between the channels A,B following every iteration. The data frame transmitted via the channelA is error-free after two iterations at the time instant t_(A2).Subsequent thereto, transfer takes place to the channel B and thecomputing capacity of the DSP still available within the TTI frame Tunder consideration is fully used for decoding the data frametransmitted via the channel B.

FIG. 7 shows a third preferred exemplary embodiment of the methodaccording to the invention. Said exemplary embodiment is described asZ⁺-strategy. The Z⁺-strategy is essentially similar to the Z-strategyshown in FIG. 5. The Z⁺-strategy differs from the Z-strategy, however,in that the decoding method is started on that channel via which thereceived signal 21 is transmitted with the best quality factor. This hasthe advantage that, in particular, if switchover takes place betweenmore than two channels within the framework of the decoding method, asmany data frames as possible, namely the data frames least subject tointerference, can be decoded until error-free within the TTI frame Tavailable.

Items of information about the quality factor of the transmittedreceived signal 21 can be obtained from various sources. Thus, forexample, it is conceivable to determine the quality factor of thereceived signals 21 on the basis of a signal-to-noise ratio(signal-to-interference ratio, SIR) of the received signals 21. In UMTSradio telecommunications systems, the SIR is determined as standard, forexample for the purpose of matching the transmitting power of themobile-radio units. The SIR can be used without great effort to selectthe first channel for performing the method according to the invention.The quality factor of the received signal may, however, also bedetermined on the basis of the bit error rate of pilot bits in adedicated physical control channel (DPCCH) of the received signals 21.

In the exemplary embodiment from FIG. 7, the decoding device 20 is firstinitialized at the start of the TTI frame T for the time periodt_(init). Subsequent thereto, that channel is selected from the twochannels A, B via which the received signal 21 is transmitted with thebetter quality factor. The selection of the channel is symbolized by anarrow 70 and is implemented in the decoding device 20 by the unit 22. Inthe present exemplary embodiment, the received signal 21 is transmittedvia channel B with the best quality factor and the decoding methodbegins with channel B. After three iterations B1 to B3, the data frametransmitted-via the channel B is error free at the time instant t_(B).Switchover then takes place to channel A, where the data frametransmitted via the channel A is decoded. The data frame of the channelA is error free after five iterations Al to A5 at the time instantt_(A1). The DSP is in the idle state within the TTI frame T underconsideration for a further two iterations.

To decode the data frame present in the subsequent TTI frame T of thechannels A, B, an initialization t_(init) of the decoding device 20 isagain first executed. A selection 70 than takes place of that channelvia which the received signal 21 is transmitted with the best qualityfactor. In the TTI frame T now under consideration, the signal 21 isbest transmitted via the channel A and the decoding method starts withthe channel A. The data frame transmitted via the channel A iserror-free after two iterations A1, A2, at the time instant t_(A2).Switchover takes place to the channel B, where the decoding of the dataframe transmitted via the channel B is started.

1. Iterative method of decoding received signals transmitted in dataframes via a plurality of channels by means of a digital signalprocessor, wherein the plurality of channels are assigned to the digitalsignal processor, starting with a first channel, a quality of a decodedsignal transmitted via the first channel is checked following everydecoding iteration, and the digital signal processor is switched over toat least one further channel of the plurality of channels if a specifiedswitchover condition is fulfilled, wherein a cyclic redundancy check isperformed for every decoded data frame transmitted via the first channelfollowing every decoding iteration; wherein the switchover condition isthat a decoded data frame transmitted via the first channel iserror-free; wherein a first number of decoding iterations for the firstchannel is specified; and wherein the digital signal processor isswitched over to at least one further channel of the plurality ofchannels if a specified first number of every decoding iterations hasbeen performed for a decoded data frame of the first channel. 2.Iterative method of decoding received signals transmitted in data framesvia a plurality of channels by means of a digital signal processor,wherein the plurality of channels are assigned to the digital signalprocessor, and wherein the digital signal processor switches overbetween a first channel and at least one further channel following everydecoding iteration for a first specified number of decoding iterations;and wherein switchover is terminated if a specified quality of thedecoded signal transmitted via the first channel exists.
 3. Methodaccording to claim 2, wherein for every decoded data frame of the firstchannel, before the digital signal processor is switched over to atleast one further channel of the plurality of channels, a cyclicredundancy check is performed following every decoding iteration; andwherein the specified quality of the decoded signal transmitted via thefirst channel is the decoded signal being error-free.
 4. Methodaccording to claim 2, wherein the iterative method is terminated if adecoded signal transmitted via a specified further channel has aspecified quality.
 5. Method according to claim 2, wherein the method isterminated if a specified second number of every decoding iterations hasbeen performed for a decoded data frame of the first channel and the atleast one further channel.
 6. Method according to claim 2, wherein fromthe at least one further channels, the channel with a best qualityfactor is selected as a first channel for transmitting the decodedsignal.
 7. Method according to claim 6, wherein the best quality factoris determined by a signal-to-interference ratio.
 8. Method according toclaim 6, wherein the best quality factor is determined on a basis of abit error rate of signals in a control channel.
 9. Method according toclaim 8, wherein the best quality factor is determined on the basis of abit error rate of pilot bits in a dedicated physical control channel(DPCCH) of the signals.
 10. A device for decoding, by means of aniterative method, received signals transmitted in data frames via aplurality of channels by a digital signal processor, comprising: aplurality of channels assigned to the signal digital processor, a meansfor switching over the digital signal processor between a first channeland at least one further channel following every decoding iteration fora first specified number of decoding iterations; and a means forterminating the switchover if a specified quality of the decoded signaltransmitted via the first channel exists.
 11. Device according to claim10, further comprising: at least two constituent decoders connected inseries, an interleave unit connecting an output of a first of a pair ofconstituent decoders to an input of a second of the pair of constituentdecoders in series, a feedback from an output of a last constituentdecoder to an input of a first constituent decoder, and a de-interleaveunit connected in series in the feedback.
 12. System for receivingreceived signals transmitted in data frames having at least one devicefor decoding the received signals by means of an iterative method,comprising: a digital signal processor, wherein a plurality of channelsare assigned to the digital signal processor, and a means for switchingover the digital signal processor between a first channel and at leastone further channel following every decoding iteration for a firstspecified number of decoding iterations; and a means for terminating theswitchover if a specified quality of the decoded signal transmitted viathe first channel exists.
 13. A radio telecommunications systemcomprising: a multiplicity of mobile-radio units, at least two basestations, the at least two base stations being in radio contact with themultiplicity of mobile-radio units, and at least one control device forcontrolling the radio telecommunications system, wherein the at leastone control device is connected to the at least two base stations, atleast one of the base stations has an at least one device for receivinga received signal transmitted in data frames, and said at least one basestation has a system according to claim
 12. 14. Radio telecommunicationssystem comprising: a plurality of base stations, a mobile-radio unit inradio contact with the plurality of base stations, wherein the pluralityof base stations, that are in radio contact with the mobile-radio unit,transmit a quality factor of a radio link to at least one controldevice, the at least one control device communicates items ofinformation about the quality factor of the radio links to at least oneof the plurality of base stations, and the at least one of the pluralityof base stations use said items of information in performing aniterative decoding by a digital signal processor, wherein a plurality ofchannels are assigned to the digital signal processor, said digitalsignal processor has a means for switching over between a first channeland at least one further channel following every decoding iteration fora first specified number of decoding iterations; and a means forterminating the switchover if a specified quality of the decoded signaltransmitted via the first channel exists.
 15. Radio telecommunicationssystem according to claim 14, wherein: at least one of the plurality ofbase stations use the items of information about the quality factor ofthe radio links to select the first channel from a number of channels,where the number of channels is specified.
 16. Radio telecommunicationssystem according to claim 14, wherein the base stations use the items ofinformation about the quality factor of the radio links to select asequence of further channels from a number of channels.
 17. Radiotelecommunications system according to claim 14, wherein at least one ofthe plurality base stations uses the items of information about thequality factor of the radio links to select an instant in time to changeto a further channel.
 18. Radio telecommunications system according toclaim 14, wherein at least one of the plurality base stations uses theitems of information about the quality factor of the radio links toselect an instant in time for termination of the iterative decodingmethod.
 19. An iterative decoding system having a multiplicity ofchannels comprising: an unpuncturing and demultiplexing unit, whereinthe unpuncturing and demultiplexing unit receives an input signaltransmitted in data frames and unpunctures the data frames, whereinunpuncturing comprises insertion of bits, with an amplitude of zero,into the transmitted data frame; the unpuncturing and demultiplexingunit being connected to a first decoding unit; the first decoding unitbeing connected to a first interleave unit; the first interleave unitbeing connected to a second decoder; the second decoder being connectedto a cyclic redundancy check device, wherein a decoded data is checkedfor errors; the cyclic redundancy device being connected to a firstscanning block and a second scanning block, wherein decoded data witherrors is output to the first scanning block and decoded data withouterrors is output to the second scanning block; the first scanning blockbeing connected to a third scanning block and to the second scanningblock, wherein the first scanning block outputs decoded data that hasbeen decoded a first number of iterations to the second scanner andoutputs decoded data that has not been decoded the first number ofiterations to the third scanning block; wherein an iteration comprisesdata transmission through the first decoder, the interleave unit, andthe second decoder; the third scanning block being connected to thesecond scanning block and a first de-interleave unit, wherein the thirdscanning block outputs decoded data that has not been decoded a secondnumber of iterations to the de-interleave, and outputs decoded data thathas been decoded a second number of iterations to the second scanningblock; the first de-interleave unit being connected to the firstdecoder; the second scanning block having a first output and a secondoutput, wherein the first output ends the iterative decoding, and thesecond output is connected to a hard conversion unit, decoded data thathas been decoded a second number of iterations is output through thefirst output and decoded data that has not been decoded the secondnumber of iterations is output through the second output; the hardconversion unit being connected to a second de-interleave unit, whereinthe hard conversion unit converts decoded data from the second decoder;the second de-interleave unit being connected to a switchover unit; theswitch over unit being connected to the unpuncture and demultiplexingunit.